The causes of brown skin blotches include skin diseases, medications and ultraviolet light, according to About.com. Other causes of brown skin blotches include liver disease, pregnancy, hemochromatosis, Addison's disease and pituitary tumors. Dark spots on the skin most commonly appear during middle age.
There are several skin diseases that may cause dark spots on the skin, and one of the causes of these is post-inflammatory hyperpigmentation, as explained by About.com. This is a dark spot that occurs following trauma to the skin, especially after infections such as acne. Other skin conditions that cause dark spots include Riehl's melanosis, melasma, poikiloderma of Civatte, linea fusca and erythromelanosis follicularis. Post inflammatory hyperpigmentation may also cause these spots.
Certain medications may cause the skin to become sensitive to the sun, and other medications may cause brown spots on the skin without sun exposure, as About.com reports. The most common medications that cause dark spots on the skin include amiodarone, estrogen, phenytoin, tetracyclines and sulfonamides. Phenothiazines may also cause brown spots to form on the skin.
Not only is ultraviolet light from the sun a cause for dark spots on the skin, but it also may cause other types of skin damage, as detailed by About.com. Ultraviolet light may cause several types of brown spots including ephelides, melasma and solar lentigines, which are freckles.